The present invention relates to a cleaning method for cleaning floating filter medium of biological filtering apparatus for use in purification treatment of night soil, domestic miscellaneous wastewater, or the like.
A feature of biological filtering apparatus is that a required volume of a treating tank therefor can be of a reduced size as compared with conventional contact-type aerobic treating apparatus or the like, so that treating apparatus can be notably reduced in size.
In such biological filtering apparatus, usually, a filter layer is formed by filling a treating tank with a filter medium which, because it has a specific gravity smaller than 1.0, floats in water. The filter medium is adhered with sludge containing aerobic or anaerobic microorganisms. As wastewater is passed through the filter layer, organic pollutants in the wastewater are treated and suspended solids are separated therefrom at the same time.
When a filtering treatment of the wastewater with the biological filtering apparatus thusly continues for a certain time, some organic pollutants accumulate in the filter layer as sludge, while suspended solids similarly become fixed and remain in the filter layer. As a result, resistance of the filter layer to wastewater flow passing therethrough gradually increases, and biological filtering effectiveness drops.
Accordingly, it is necessary to prevent complete closure of the filter layer by periodically cleaning the accumulated excess sludge and suspended solids and discharging them outside the system; thus, generally, cleaning water is passed through the filter layer in a downward flow at a constant flow rate, and the sludge is washed away while "spreading out" the filter layer with the cleaning water flow.
In the case of, however, a filter medium of an extremely small specific gravity, such as a foamed plastic filter medium, the filter layer is not sufficiently spread unless a flow velocity of the cleaning water is quite fast; as a result, a huge volume of cleaning water is needed, and an overall treating capacity of the biological filtering apparatus is lowered.
To solve these problems, various methods have, hitherto, been developed.
For example, (1) agitating blades have been provided beneath a filling layer (filter layer) formed in a treating tank, for mechanically spreading the filter layer during the cleaning operation (Japanese Laid-open patent 3-242294); and (2) a floating filter medium has been gradually and continuously drawn from inside to outside a treating tank, where it has been cleaned and then put back into the treating tank (Japanese Patent Publication 63-21553).
In the respective methods (1) and (2), the agitating blades have been installed in the treating tank, and the circulating and cleaning mechanism has been provided outside the tank. These treating apparatus themselves have been large in size, and operation and maintenance thereof have required much labor.
In a system which attempts to solve these problems, as shown in FIG. 10, a draft tube 2 and compressed-air feed pipe 3 are disposed in a treating tank 1 which has a crude water feed pipe 7 and a wastewater discharge pipe 8. Air 5 is injected from the compressed air feed pipe 3 when a filter layer 4 is being cleaned in order to generate an ascending stream 6 of wastewater in the draft tube 2, so that the wastewater in the treating tank 1 is caused to flow in the arrow direction; thereby forcefully spreading the filter layer 4 (Japanese Patent Publication 62-13043).
In such technology, however, with the wastewater being caused to flow by the ascending stream 6 alone, it is difficult to completely separate and remove the sludge and solid matter affixed to the filter medium 4a by sufficiently spreading the filter layer 4, and, in the case of a very light filter medium 4a such as foamed styrol, it is impossible to completely clean it. Hence, a circulation time of the flowing wastewater must be unduly long. After the circulation is stopped, in order to cause the sludge to settle, a cleaning water discharge valve 9 is opened, and the filter medium 4a is cleaned again by the supernatant in the treating tank 1.
As a result, a huge volume of cleaning water is needed, and the cleaning takes much time and labor, while various problems arise.
It is an object of this invention to provide a method for cleaning a floating filter medium for biological filtration, that: (1) does not require a mechanism for cleaning the filter medium separately inside or outside a treating tank, which, in turn, requires an unduly large purifying apparatus; (2) does not require the fluidization, or flowing, of wastewater by air pump alone, which makes it difficult to sufficiently spread a filter layer when the floating filter medium is particularly light in weight, and the affixed matter cannot be sufficiently separated and removed; and (3) does not require a huge volume of cleaning water for cleaning a filter medium so that overall treating performance of the wastewater treating apparatus is decreased.
Further, it is an object of this invention to provide a method for cleaning a floating filter medium for biological filtering apparatus capable of cleaning, almost completely, even a lightweight filter medium with a small volume of cleaning water.